1. Field of the Invention
The invention concerns a method to correct imaging errors (in particular distortions) in an x-ray image intensifier system of the type having an image converter tube to convert incident x-ray radiation into visible light and a digital camera system that is optically downstream of the image converter tube, wherein the camera system has an image sensor to convert the incident visible light into digital images, and wherein an electronic image processing unit is provided for post-processing the digital images. The invention furthermore concerns an associated x-ray image intensifier system.
2. Description of the Prior Art
An x-ray image intensifier is preferably used in medical x-ray diagnostics, as well as in materials testing. It converts x-ray radiation incident in an electron tube (also designated as an image converter tube, shortened to image converter or image intensifier) into visible light and amplifies the signal, analogous to an optical image intensifier. A mapping of the x-ray input window to a (usually) smaller optical output window thereby ensues. The enlargement or reduction of the image from the input screen to the output screen can be controlled via suitable selection of the parameters of the image intensifier (for example acceleration voltage, electron optics). This is also called “zoom”. The visible light exiting from the output window of the image converter tube is imaged by optics on the image receiver or image sensor of a digital camera and is there transduced into electronically processable digital images or video sequences that can be displayed on a display of an associated display device.
Before being shown on the display, the raw data of the image receiver or image sensor are typically processed in an electronic image processing unit. Various image processing algorithms are typically applied to the camera image in the image processing unit, for example dark current correction, light image correction, sharpness correction, rotation, etc. The design of the image processing depends on the usage purpose of the stated functions. Not all of the aforementioned functions must be realized. The image processing can be integrated into the camera (and therefore into the image intensifier system) or can be arranged separately. It is also possible that only some functions are realized internally in the camera but other functions are external. Given simple tasks, the image processing unit can also be omitted entirely, but for most applications at least rudimentary image processing functions are provided.
The image converter tube of the x-ray image intensifier is based on the principles of electron optics. It is therefore very sensitive to a magnetic field since this deflects the electrons from their path by Lorentz force. This manifests itself in imaging errors, in particular in image deformations or distortions (for example pillow-shaped or barrel-shaped distortions). The problem to be solved is to avoid or to compensate for the image deformations generated by a magnetic field. The Earth's magnetic field already has a visible influence on the image deformation. Therefore corresponding measures can normally not be omitted.
One approach is to shield the image intensifier from the magnetic fields. For example, material known as μ-metal is used for this that forms a shell around the image intensifier. This approach has previously been predominantly used, also in connection with other solution approaches presented in the following. The input window represents the largest problem with this approach. In this region the use of a shielding material in the necessary material thickness is not possible since the high x-ray absorption in the metal foil would require a significant increase of the incident x-ray intensity that would be necessary to achieve an acceptable image quality. This is either not possible in principle without additional measures or is very undesirable (for instance in medical x-ray diagnostics) because this would lead to a severely increased patient dose. For this reason only the lateral surfaces of the x-ray image intensifier are normally sufficiently strongly shielded, but the input window is not, or is only very weakly, shielded. In all cases, foils with a thickness of 25 μm are used in this region that are very complicated and expensive to produce.
In addition or as an alternative to shielding with μ-metal, there is the approach to compensate for the external magnetic fields with a suitable generated counter-field. By the use of a coil arrangement around the image intensifier, an additional magnetic field is generated that cancels the effect of the Earth's magnetic field on the image. For this purpose, the external magnetic field is measured by a magnetic field probe and a matching current for the coil arrangement is generated by an electronic unit. Equipping of the image intensifier with suitable coils and with a reliable arrangement for charging them with current is relatively complicated and cost-intensive. In particular in non-stationary systems, the magnetic field (and therefore the type and the severity of the image deformation) can change very quickly depending on the travel path and alignment of the system in space.
Even without a magnetic field, an image intensifier exhibits an image deformation. The optics following the image intensifier can likewise deform the image. These static deformations should likewise be suppressed or compensated if possible.